The power output requirements for the next generation of magnetic resonance imaging scanners is a substantial performance step up from the current available scanners. A doubling of the gradient strength is desired with a simultaneous increase in duty cycle. In some cases the desired increase in duty cycle is as large as a five to one ratio with the new objective being 75 percent. No increases in the slew rate or change in magnetic field per unit of time (dB/dt) will be permitted because of the human limitations resulting from muscle twitch and eventually associated pain that results from internally produced voltages on the nervous system. Additionally, there is the ever increasing pressure to contain health care costs which limits the amount of money available to improve on technology.
Presently there is an inherent limitation in the available operating voltages of high-voltage semi-conductors which in turn limits the available output voltage of the gradient amplifiers which drive the coils of the imaging scanners. If the coils are redesigned for lower voltage requirements, the resulting field will not have the desired resolution as there will be too few current carrying conductors or turns in the coil. Heretofore several approaches have been used to increase the available coil voltage. One such approach was to switch a high voltage supply in series with the existing gradient amplifier to counter high voltage requirements during a ramp of current into the coil. After the ramp has passed, the power supply is removed from the circuit leaving only the gradient amplifier in control of the coil. In the accompanying figures (FIG. 1 and FIG. 2) two prior art additional approaches are illustrated. One (FIG. 2) utilizes a split gradient coil system in which two gradient amplifiers 20, 22 per axis of the scanner are associated with the gradient coil 18 which is split into two halves per axis. Each half would require only half the total voltage required by high speed imaging. This reduces the required voltage of the gradient amplifiers to one half of the requirement that occurs when the gradient coils are placed in series. In this approach, the system lacks the ability to place the coils in series and thus there is no way to shift between a high dB/dt and a high dB/dx which is representative of the change in magnetic field per unit distance.
Another approach which has been used incorporates a dual configuration single gradient coil system. This system as illustrated in the accompanying prior art drawing (FIG. 1) uses a single gradient coil 10 per axis of the imaging scanner and two gradient amplifiers 12, 14 for each such axis. The two amplifiers can be used either for increased (double) voltage by placing them in series (a high dB/dt) or by placing them in parallel for increased (double) current for a high dB/dx using switch 16. In this dual configuration system, when two gradient amplifiers are used for single gradient coil and the amplifiers share a single power supply, the amplifiers will be restricted to a half-bridge topology which can only produce one half of the total supply voltage per amplifier. The semi-conductors or amplifiers of this half bridge are then exposed to the full supply voltage. This problem of exposing the semi-conductors to a full voltage supply can be solved by utilizing the full bridge with a separate floating power supply for each amplifier. This results in a costly solution since there must be individual power supplies for each gradient system.